1. Field of the Invention
The present invention relates generally to methods and systems for production of hydrocarbons, hydrogen, and/or other products from various subsurface formations such as hydrocarbon containing formations.
2. Description of Related Art
Hydrocarbons obtained from subterranean formations are often used as energy resources, as feedstocks, and as consumer products. Concerns over depletion of available hydrocarbon resources and concerns over declining overall quality of produced hydrocarbons have led to development of processes for more efficient recovery, processing and/or use of available hydrocarbon resources. In situ processes may be used to remove hydrocarbon materials from subterranean formations that were previously inaccessible and/or too expensive to extract using available methods. Chemical and/or physical properties of hydrocarbon material in a subterranean formation may need to be changed to allow hydrocarbon material to be more easily removed from the subterranean formation and/or increase the value of the hydrocarbon material. The chemical and physical changes may include in situ reactions that produce removable fluids, composition changes, solubility changes, density changes, phase changes, and/or viscosity changes of the hydrocarbon material in the formation.
Large deposits of heavy hydrocarbons (heavy oil and/or tar) contained in relatively permeable formations (for example in tar sands) are found in North America, South America, Africa, and Asia. Tar can be surface-mined and upgraded to lighter hydrocarbons such as crude oil, naphtha, kerosene, and/or gas oil. Surface milling processes may further separate the bitumen from sand. The separated bitumen may be converted to light hydrocarbons using conventional refinery methods. Mining and upgrading tar sand is usually substantially more expensive than producing lighter hydrocarbons from conventional oil reservoirs.
Typical temperature measurement methods may be difficult and/or expensive to implement for use in assessing a temperature profile of a heater located in a subsurface formation for heating in an in situ heat treatment process. The desire is for a temperature profile that includes multiple temperatures along the length or a portion of the heater in the subsurface formation. Thermocouples are one possible solution; however, thermocouples provide only one temperature at one location and two wires are generally needed for each thermocouple. Thus, to obtain a temperature profile along a length of the heater, multiple pairs of wires are needed. In some cases, the wellbore for the heater may not be large enough to accommodate the number of wires needed for adequate temperature monitoring. The risk of failure of one or more of the thermocouples (or their associated wires) is increased with the use of multiple wires in the subsurface wellbore. In addition, thermocouples installed in high temperature applications (>300° C.) may suffer from a phenomenon known as temperature measurement drift. Temperature measurement drift may be a significant source of error.
Another possible solution is the use of a fiber optic cable temperature sensor system. The fiber optic cable system provides a temperature profile along a length of the heater. Commercially available fiber optic cable systems, however, typically only have operating temperature ranges up to about 300° C. and are susceptible to mechanical damage because of the softening of the fiber and/or fiber coatings resulting in the various materials adhering to each other. Thus, these systems are not suitable for measurement of higher temperatures encountered while heating the subsurface formation during the in situ heat treatment process. Some experimental fiber optic cable systems are suitable for use at these higher temperatures but these systems may be too expensive for implementation in a commercial process (for example, a large field of heaters). Thus, there is a need for a simple, inexpensive system that allows temperature assessment at one or more locations along a length of the subsurface heater used in the in situ heat treatment process.